As a measure of the purifying capability of a catalyst for purifying exhaust gas from a gasoline engine (hereinafter referred to as the catalyst), the oxygen storage capability of the catalyst has been heretofore noted. It is known that the deterioration degree of the catalyst is estimated by measuring the oxygen storage capability with an oxygen sensor. The deterioration degree is the amount by which a catalyst has deteriorated, that is, by how much it has lost its effectiveness, from use over time. Examples of a method of detecting the catalyst deterioration include the following:
First Catalyst Deterioration Detecting Method
For example, in a case where an air/fuel ratio is controlled, via a carburetor or fuel injector, or by addition/reduction of air via a catalyst air pump, based on an output of an oxygen sensor disposed downstream from the catalyst, the deterioration degree of the catalyst can be estimated based on the output of the oxygen sensor. Note that the output voltage is inversely proportional to the oxygen at the sensor.
Specifically, as shown in FIG. 7A, at a time when the voltage output of the oxygen sensor downstream from the catalyst rises, the air/fuel ratio is controlled toward a lean side. At a time when the output of the oxygen sensor falls, the air/fuel ratio is controlled toward a rich side. Here, when the purifying efficiency of the catalyst is high, even if the air/fuel ratio is controlled toward the lean side when the output of the oxygen sensor downstream from the catalyst rises, the oxygen storage capability of the catalyst is high, so that oxygen is stored. Therefore, the output voltage of the oxygen sensor downstream from the catalyst still remains high. The output voltage does not drop until oxygen is sufficiently stored. Subsequently, when the output voltage lowers, the air/fuel ratio is controlled to the rich side. Since the stored oxygen is consumed, the output voltage of the oxygen sensor downstream from the catalyst still remains low. The output voltage does not increase until the stored oxygen is consumed. As aforementioned, when the purifying efficiency of the catalyst is high, a reversing time, i.e., a high-output keeping time plus a low-output keeping time is lengthened. When the purifying efficiency of the catalyst is lowered, however, the oxygen storage capability of the catalyst is lowered. Therefore, the reversing time is shortened as shown in FIG. 7B. Therefore, the deterioration degree of the catalyst can be detected by tracing the output voltage of the oxygen sensor downstream from the catalyst and judging whether the reversing time is long or short.
Second Catalyst Deterioration Detecting Method
In a case where the air/fuel ratio is controlled based on an output of an oxygen sensor disposed upstream from the catalyst, the deterioration degree of the catalyst is estimated based on an output of an oxygen sensor disposed downstream from the catalyst.
Specifically, when the purifying efficiency of the catalyst is high, the oxygen storage capability of the catalyst is high. Therefore, the change of the air/fuel ratio toward the rich/lean side in the exhaust gas before passing through the catalyst, i.e., the change of an oxygen partial pressure, is moderated by passing the exhaust gas through the catalyst. Specifically, as shown in FIG. 8A, irrespective of whether the air/fuel ratio of the exhaust gas before passing through the catalyst is rich or lean, the oxygen partial pressure of the exhaust gas after passing through the catalyst is reduced. The amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst is reduced. However, when the purifying efficiency of the catalyst is lowered, the oxygen storage capability of the catalyst is lowered. Therefore, even after the exhaust gas is passed through the catalyst, the change of the air/fuel ratio to the rich/lean side in the exhaust gas before passing through the catalyst is kept as it is and fails to be moderated. Specifically, as shown in FIG. 8B, the change of the air/fuel ratio to the rich/lean side in the exhaust gas before passing through the catalyst results in the change in the oxygen partial pressure of the exhaust gas after passing through the catalyst. The amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst is increased in the same manner as in the front oxygen sensor. Therefore, the deterioration degree of the oxygen storage capability of the catalyst can be detected by tracing the change of the output voltage of the oxygen sensor downstream from the catalyst and judging whether the amplitude of the output voltage wave form is large or small.
However, in a case where the deterioration degree of the catalyst for an engine using compressed natural gas or CNG fuel or the like is estimated in the same manner as the first or second catalyst deterioration detecting method, defects arise and the catalyst deterioration cannot be detected.
Specifically, even when the purifying ratio of the catalyst is high, that is, when the catalyst has not deteriorated, in the first catalyst deterioration detecting method, as shown in FIG. 7C, the reversing time of the output voltage of the oxygen sensor downstream from the catalyst is shortened in a certain temperature range in the same manner as when the catalyst has deteriorated, because of the influence of a large amount of methane contained in the CNG fuel. Furthermore, in the second catalyst deterioration detecting method, as shown in FIG. 8B, the problem is that the amplitude of the output voltage wave form of the oxygen sensor downstream from the catalyst changes in the same manner as when the catalyst has deteriorated.
More specifically, since the methane contained in the exhaust gas is not sufficiently burnt even after passing through the catalyst, unburnt methane remains. When a detection electrode of the oxygen sensor downstream from the catalyst has a low temperature, however, the unburnt methane does not react with oxygen in the vicinity of the detection electrode. Therefore, no change occurs in the oxygen partial pressure, and the output voltage of the oxygen sensor downstream from the catalyst is not influenced.
However, in the first catalyst deterioration detecting method, when the temperature of the detection electrode of the oxygen sensor downstream from the catalyst reaches or exceeds a certain temperature, the unburnt methane causes a burning reaction with the oxygen on the detection electrode. Therefore, a difference in oxygen concentration between a reference electrode and the detection electrode changes in accordance with the concentration of methane. If the amount of methane exceeds the stoichiometric amount at a time when methane causes a burning reaction with the oxygen in the exhaust gas, the oxygen of the detection electrode is drawn away. Therefore, the output voltage is largely raised. If the amount of methane is equal to or less than the stoichiometric amount, no oxygen at the detection electrode is drawn away. Therefore, the output voltage is lowered. As a result, the reversing time depends on the methane concentration, but does not depend on the oxygen storage capability of the catalyst. The burning reaction becomes significant as the temperature of the detection electrode rises. Therefore, the reversing time of the oxygen sensor downstream from the catalyst becomes shorter as the temperature of the detection electrode rises.
Also, in the second catalyst deterioration detecting method, when the temperature of the detection electrode of the oxygen sensor downstream from the catalyst reaches or exceeds a certain temperature, the unburnt methane causes a burning reaction with the oxygen at the detection electrode. Since the oxygen at the detection electrode is drawn away, a difference in the oxygen partial pressure is generated. The output voltage is largely raised in accordance with the methane concentration, i.e., when the methane concentration is high or the air/fuel ratio is rich. For this reason, even if the catalyst is normal, the output voltage of the oxygen sensor downstream from the catalyst changes in accordance with the change of the air/fuel ratio toward rich/lean. Therefore, the catalyst deterioration cannot be detected.
As aforementioned, in the case where the deterioration degree of the catalyst for the engine using the CNG fuel or the like is detected based on an output signal of the oxygen sensor downstream from the catalyst, a problem remains unsolved in that the output voltage of the oxygen sensor downstream from the catalyst is not stabilized because of the burning reaction of the oxygen in the vicinity of the detection electrode with the unburnt methane.